1. Field of the Invention
The present invention generally relates to a microwave plasma generating apparatus and, more particularly, to the microwave plasma generating apparatus of a simple structure wherein plasma generated as a result of microwave radiation can be positioned at a predetermined location.
2. Description of the Prior Art
A microwave plasma generating apparatus is well known which comprises a microwave oscillator for generating a microwave, a microwave guide for guiding the microwave therethrough, and a discharge tube containing a source gas. This known microwave plasma generating apparatus is so designed and so operated that plasma can be generated when the source gas within the discharge tube is radiated with the microwave generated by the microwave generator and subsequently guided through the microwave guide. In this known microwave plasma generating apparatus, energies of microwaves can be absorbed by the plasma generated within the discharge tube with the consequence that the microwave power can be maximized at a region of the plasma adjacent a microwave inlet.
Accordingly, once the microwave power is maximized, at the specific region adjacent the microwave inlet, the position at which the plasma is generated tends to shift towards a specific region and, as a result of the shift in position, the plasma is brought into contact with the discharge tube causing the latter to rupture.
Another type of microwave plasma generating apparatus is well known wherein, a plasma generating chamber is used in place of the plasma discharge tube. The chamber has a crystal pane through which a microwave is introduced into the chamber. In this type of microwave plasma generating apparatus, by a similar reason as discussed above, the position at which the plasma is generated tends to shift to the area where an electric field developed by the microwave is intensified. This shift in area of origin of the plasma causes the plasma to contact the crystal pane and, by the action of heated evolved by the plasma, the crystal pane tends to be thermally broken.
Where the microwave plasma generating apparatus is used as a plasma CVD (chemical vapor deposition) device, a substance produced by plasma reaction is often deposited on the crystal pane during the chemical vapor deposition. The consequence is that the microwave tends to be absorbed by the substance or semiconductor substance which has been deposited on the crystal pane, resulting in a reduction of the efficiency of propagation of the microwave towards the plasma generating chamber and, in the worst case it may happen, the plasma will fail to generate.
To obviate the foregoing problems, an attempt has been made to provide a sophisticated microwave plasma generating apparatus which comprises a microwave oscillator, a tubular microwave guide through which a microwave generated by the microwave oscillator propagates, a discharge tube operable to produce a plasma when a source gas introduced into the discharge tube is radiated with the microwave guided through the tubular microwave guide and into the discharge tube, and a tuner or movable stub turner operatively coupled with the discharge tube and having a short plunger which can reflect the microwave emitted from that exit of the tubular microwave guide which is coupled with the discharge tube and introduced into the discharge tube. The short plunger used in this microwave plasma generating apparatus includes a baffling or reflecting plate means supported for reciprocating motion in a direction generally parallel to the direction of propagation of the microwave.
This sophisticated microwave plasma generating apparatus is so designed that, when the baffling or reflecting plate means disposed so as to allow the discharge tube to locate between the opening of tube and the opening of wave guide reflects the incoming microwave, a reflected component of the microwave can interfere with the remaining microwave to provide a standing microwave at a predetermined area within the discharge tube thereby to fix the area of origin of the plasma within the discharge tube.
However, the employment of the baffling or reflecting plate means movable in a direction parallel to the direction of propagation of the microwave has been found problematic in that an increase of the applied electric power results in a shift of the plasma towards a source of microwave, that is, towards the microwave oscillator, and even in the case where no large amount of electric power is applied, the plasma tends to move slowly, but steadily towards the source of the microwave.
Accordingly, where a large amount of electric power is employed to generate the plasma and/or where the generation of the plasma is retained for a substantially prolonged length of time, it often occurs that the transmission force of the microwave is lowered consequent upon rupture of the wall defining the discharge tube and/or deposition of the plasma reaction product on the crystal pane.
It is to be noted that the microwave plasma generating apparatus of the type utilizing the short plunger requires the use of a drive system for driving the baffling or reflecting plate means in the direction generally parallel to the direction of propagation of the microwave and, therefore, it has an additional problem in that the device itself tends to become bulky.